1. Field of the Invention
The field of the present invention relates generally to the recovery of drilling fluids from discharge cuttings fluids from a drilling/production operation, more particularly, a method utilizing various types of presses for the recovery of such drilling fluids through compaction and defluidization of entrained solids in a cuttings slurry prior to such cuttings being injected into a well casing or in conjunction with other environmental distribution and/or disposal operations.
2. General Background
In oil well drilling operations, drilling fluid containing additives is circulated downwardly through the drill string to lubricate and remove cuttings from the bit. A mixture containing drilling fluid and cuttings is then returned to the surface through and annulus around the drill pipe. "Adherent drilling fluid" is defined as drilling fluid adhering to the drill cuttings, and, if the drilling fluid is oil-based, the adherent drilling fluid also includes oil.
It is well known that the drill cuttings must be separated from the drilling fluid so that the drilling fluid can be recirculated. Additionally, solid cuttings generated in a drilling process, such as during exploration for oil or gas, which have been contaminated with adherent drilling fluid must be cleansed to remove surface contaminates prior to discharge of the cuttings to the environment. Several such methods and apparatus are disclosed by U.S. Pat. Nos. 5,361,998, 5,303,786, 5,129,468, and 4,546,783. Such apparatus are particularly beneficial in laundering or cleansing of drill cuttings on offshore drill platforms so that the drill cuttings are environmentally safe for discharge into the sea. However, the loss of a portion of the adherent fluids is inevitable and is becoming more of a concern.
Many of the problems associated with drilling fluid recovery for onshore operations are expressed by Hart in U.S. Pat. No. 5,330,017. Hart suggests that due to environmental concerns much of the slurry is transported in a fluid or semi-fluid state to approved disposal sites. Such sites utilize deep wells whereby hazardous waste can be injected back into the earth or mixed with chemicals such as lye and fly ash which render the materials acceptable for land reclamation. Disposal sites may also provide centrifuges as a means of defluidizing the slurry and rely heavily on polymers added to the effluent to render the discharge liquids safe for reintroduction into the environment.
Many recovery and treatment apparatus utilize separate cells having low speed agitators to stir a mixture of cutting and cleansing solution called surfactants. The cuttings are transferred from one cell to the next where additional agitation and cleansing takes place. Thereafter, a slurry of cleansed drill cuttings and surfactant is pumped from the cells to a vibrating screen operation whereby most of the surfactant is removed and sent back to the system. In some cases a portion of the surfactant solution, which is rich in fine drill cuttings and adherent drilling fluids, is run through one or more hydrocyclone separators which discharge the fine drill cuttings in solution separated from the larger, cleansed drill cuttings. However, it has been the practice in the past to simply pass the cuttings over one or more vibrating screens to recover the majority of the drilling additives and discharge the remainder as waste material. In any case, it is the overflow and underflow of such discharge slurries comprising surfactant solution, drilling fluids and entrained fine drill cuttings which is the focus element of the present invention.
As discussed by Lott in U.S. Pat. No. 4,546,783, Hydrocyclones used in the recovery system tend to lose 4% of the surfactant solution alone in the process, which is environmentally and economically undesirable. An even greater percentage of drilling fluids are also lost in the process. Lott further suggested a process and apparatus for recovering more of the surfactant. However, Lott's use of a vacuum chamber and a drag link conveyer to clear additional shaker screens, the use of a second hydrocyclone, gas spargers and liquid spray nozzles to induce the entrained solids to rise to the surface in yet another decanter so that they can be drained off into a second decanter prior to disposal, seems to be an over-complication of the process. However, such drastic measures to recover only 4% of the surfactant, along with the drilling fluids, is indicative of the need for a more efficient method of recovery.
Although screw presses have been widely used in the agricultural industry to dewater fibrous slurries, such presses have not gained acceptance in the earth drilling industry for a number of reasons. Compressing earth cuttings developed from drilling operations would be difficult under most conditions, due to the volume, the abrasiveness and nonuniformity of such materials. Dewatering screw conveyors and screen conveyor systems have been used with some success in mining operations to remove a large portion of the residual water. However, the drilling additives associated with petroleum drilling operations make defluidizing more complicated.
It has been found that screw presses, such as disclosed by Eichler in U.S. Pat. No. 5,009,795, could serve as the basis for a defluidizing press in the present invention concept. However, due to the nature of the materials handled, abrasiveness and the material's lack of compressibility, a more robust screw flighting and a much finer screen is required. A means of controlling the flow of material to form compaction is also required which will not restrict the material discharge. It is also known, according to Gloacki's U.S. Pat. No. 4,709,628, that a variable damper having a conical shape can be used to control the material discharge of such screw presses. However, Glowacki uses a plurality of flaps, which would become compacted or misshape and impair the flow of heavy non-compressible materials such as earth cuttings. Therefore, a more rigid conical or elliptical shape would be more practical. It has therefore been found that a defluidizing type press designed specifically to handle a slurry of drill cuttings may be utilized to recover drilling fluids while defluidizing the discharge cuttings, thereby resulting in a savings of costly drilling additives and reducing the volume of discharge into the environment. Such savings are further enhanced as a result of a reduction in environmental additives, such as lime and fly ash, and other such chemicals used to neutralize the discharge waste material when being reintroduced into the environment. By defluidizing the discharge slurry, the volume of disposable material is reduced. Therefore less chemicals are required to treat the material before introduction into the environment.